Imaging system, and method for specifying UV emission location using same

ABSTRACT

An imaging system includes: an image sensor sensitive to ultraviolet light and visible light; a lens configured to focus light from a subject onto the image sensor; and an image processor configured to process image signals output from the image sensor. The image processor obtains the difference between image signals A 1  and A 2  output from the image sensor at times t1 and t2, respectively. If the differential signal A 3  is greater than or equal to a predetermined value, the image processor determines that light from the subject contains the ultraviolet light, and generates an image signal CI based on the differential signal A 3.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of International Application No. PCT/JP2018/5216filed on Feb. 15, 2018, which claims priority to Japanese PatentApplication No. 2017-054659 filed on Mar. 21, 2017. The entiredisclosures of these applications are incorporated by reference herein.

BACKGROUND

The present invention relates to an imaging system capable of imagingweak ultraviolet light outdoors or under an illumination environment toidentify the location generating the weak ultraviolet light, and amethod of identifying the location emitting the weak ultraviolet lightusing the imaging system.

A technique has been suggested (see e.g., Japanese Unexamined PatentPublication No. H1-152377, hereinafter referred to as Patent Document1), which simultaneously takes ultraviolet light with a wavelengthshorter than 400 nm and visible light much weaker than the ultravioletlight into an ultraviolet light imaging means, while observing thebackground with eyes, to detect partial discharge. In an attempt todetect the weak ultraviolet light with naked eyes under external lightor illumination, for example, illumination light may be an obstacle. Theweak visible light is thus taken in at the same time as the ultravioletlight to confirm the background based on the visible light. With respectto a photoelectric conversion member described in Patent Document 1, thequantum efficiencies for the visible light with the shortest wavelength(i.e., 400 nm) and a wavelength of 460 mm are lower than that for theultraviolet light by two and five orders of magnitude, respectively (seeFIG. 1 of Patent Document 1).

SUMMARY

However, a dedicated filter for cutting visible light is required todetect weak ultraviolet light under illumination containing the visiblelight, which increases the price of the device. The known art disclosedin Patent Document 1 largely reduces the sensitivity of thephotoelectric conversion member to the visible light as described aboveto detect the weak ultraviolet light under illumination containing thevisible light. If the visible light has an insufficient intensity, thebackground can be confirmed insufficiently, which leads to difficulty inidentifying the location emitting the weak ultraviolet light. Inaddition, the photoelectric conversion member detects only light in ablue band (i.e., with a wavelength around 400 nm) as the visible light,and is not sensitive to light in green and read bands with wavelengthslonger than the above-mentioned wavelength. The problem is thereforethat the known art is applicable only to limited illuminationconditions.

The present invention was made in view of the problem. It is anobjective of the present invention to provide an imaging system capableof detecting weak ultraviolet light without degrading the sensitivity tobackground visible light to identify the location emitting theultraviolet light, and a method of identifying the location emitting theultraviolet light.

In order to achieve the objective, the present invention calculates thedifference between image signals acquired at different times to obtainan image signal associated with the ultraviolet light.

Specifically, an imaging system according to the present inventionincludes: an image sensor sensitive to ultraviolet light and visiblelight; a lens configured to focus light from a subject onto the imagesensor; and an image processor configured to process image signalsoutput from the image sensor. The image processor acquires adifferential signal between an image signal output from the image sensorat a first time and an image signal output from the image sensor at asecond time, determines that the differential signal contains a signalassociated with the ultraviolet light if the differential signal isgreater than or equal to a predetermined value, and generates a firstimage signal based on the differential signal.

With this configuration, if the difference between the image signalsacquired at the different times is greater than or equal to thepredetermined value, it is determined that the differential signalcontains the signal associated with the ultraviolet light, and the firstimage signal is generated based on the difference. This allows fordetection of even weak emission of the ultraviolet light from thesubject.

In one preferred embodiment, the image processor further generates afirst composite image signal of the image signal output from the imagesensor at the first time and the first image signal or an amplifiedfirst image signal.

This configuration allows for acquisition of, for example, the outlineof the subject from the image, which mainly contains the visible light.From the outline and the image associated with the ultraviolet light,the location emitting the ultraviolet light can be identified in thesubject.

In one preferred embodiment, the imaging system further includes a focuscontroller configured to control a position of the lens to adjust afocal length of the lens with respect to light with differentwavelengths. The focus controller controls the position of the lens tomatch the focal length of the lens to the ultraviolet light at the firstand second times, and controls the position of the lens to match thefocal length of the lens to the visible light at a third time. The imageprocessor further generates a second composite image signal of an imagesignal output from the image sensor at the third time and the firstimage signal or an amplified first image signal.

This configuration allows for acquisition of clear images associatedwith the ultraviolet and visible light, which leads to more reliableidentification of the light-emitting location. In addition, the S/Nratio of the image associated with the ultraviolet light is improved toprovide a clearer image signal.

In one preferred embodiment, the imaging system further includes adisplay configured to display an image of the subject based on the firstor second composite image signal.

A method of identifying a location emitting ultraviolet light using animaging system including at least: an image sensor sensitive toultraviolet light and visible light; a lens configured to focus lightfrom a subject onto the image sensor; and an image processor configuredto process image signals output from the image sensor. The methodincludes: imaging a subject to acquire ones of the image signals at afirst time and a second time using the image sensor; obtaining adifferential signal between the ones of the image signals acquired atthe first and second times using the image processor; determiningwhether or not the differential signal is greater than or equal to apredetermined value using the image processor; determining that thedifferential signal contains a signal associated with the ultravioletlight if the differential signal is greater than or equal to apredetermined value; generating a first image signal based on thedifferential signal containing the signal associated with theultraviolet light using the image processor; and identifying thelocation emitting the ultraviolet light in the subject using the imageprocessor, based on the first image signal.

With this method, if the difference between the image signals acquiredat the different times is greater than or equal to the predeterminedvalue, it is determined that the differential signal contains the signalassociated with the ultraviolet light, and the first image signal isgenerated based on the difference. This allows for detection of evenweak emission of the ultraviolet light from the subject. Theidentification of the location emitting the ultraviolet light leads todetection of, for example, a malfunction in a facility.

In one preferred embodiment, the location emitting the ultraviolet lightis identified in the subject using the image processor, based on a firstcomposite image signal of the first image signal or an amplified firstimage signal and the one of the image signals acquired at the firsttime.

This method allows for acquisition of, for example, the outline of thesubject from the image, which mainly contains the visible light. Fromthe outline and the image associated with the ultraviolet light, thelocation emitting the ultraviolet light is identified in the subject,which leads to detection of, for example, a malfunction in a facility.

In one preferred embodiment, the method further includes: adjusting aposition of the lens to match a focal length of the lens to theultraviolet light at the first and second times; adjusting the positionof the lens to match the focal length of the lens to the visible lightat a third time; and imaging the subject to acquire an image signal atthe third time using the image sensor. The location emitting theultraviolet light is identified in the subject using the imageprocessor, based on a second composite image signal of the first imagesignal or an amplified first image signal and the image signal acquiredat the third time.

This method allows for acquisition of clear images associated with theultraviolet and visible light, which leads to more reliableidentification of the light-emitting location. In addition, the S/Nratio of the image associated with the ultraviolet light is improved toprovide a clearer image signal. This leads to more reliableidentification of the light-emitting location in the subject.

The imaging system according to the present invention allows fordetection of weak ultraviolet light and background visible light,without degrading the sensitivity to the background visible light. Theimaging system outputs an image with a high S/N ratio with respect tothe ultraviolet light. The method of identifying the location emittingultraviolet light according to the present invention allows fordetection of weak ultraviolet light from a subject outdoors or under anartificial illumination environment to clearly identify the locationemitting the ultraviolet light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an imaging system according to afirst embodiment of the present invention.

FIG. 2 is a flowchart for identifying the location emitting ultravioletlight according to the first embodiment.

FIG. 3 is a functional block diagram of an imaging system according to asecond embodiment of the present invention.

FIG. 4 is a flowchart for identifying the location emitting ultravioletlight according to the second embodiment.

DETAILED DESCRIPTION

Embodiments of the present disclosure will now be described in detailwith reference to the drawings. The following description ofadvantageous embodiments is only examples in nature, and is not intendedto limit the scope, applications or use of the present invention.

First Embodiment

FIG. 1 is a functional block diagram of an imaging system according tothis embodiment. The imaging target of this imaging system 100 isultraviolet light and visible light. In this specification, theultraviolet light has a wavelength within a range from 200 nm(inclusive) to 400 nm (exclusive), while the visible light has awavelength within a range from 400 nm to 700 nm. Alternatively, theultraviolet light may have a wavelength within a range from 300 nm to380 nm, while the visible light may have a wavelength within a rangefrom 400 nm to 650 nm.

The imaging system 100 includes a lens (condenser) 101, an image sensor102, and an image processor 103. The lens 101 collects light from asubject S. The image sensor 102 receives the light collected by the lens101, and performs photoelectric conversion. The image processor 103processes signals output from the image sensor 102.

The lens 101 is made of high-purity glass such as quartz, or a resinmaterial such as acrylic, to transmit not only visible light but alsoultraviolet light. The image sensor 102 has a photoelectric conversionfilm (not shown) with predetermined sensitivities to both theultraviolet light and visible light. The photoelectric conversion filmis made of, for example, silicon or an organic material.

The image processor 103 that processes the signals output from the imagesensor 102 is, for example, a digital signal processor (DSP), a fieldprogrammable gate array (FPGA) or a combination thereof. The imageprocessor 103 functions to store signals output from pixels of the imagesensor 102, and performs arithmetic processing. The image processor 103also functions to output the signals output from the image sensor 102and subjected to arithmetic processing as image signals.

A display 104 receives the image signals output from the image processor103, and displays the image signals as images. The display 104 may be,for example, a liquid crystal monitor or a monitor using an organic ELpanel. The display 104 may be incorporated into the imaging system 100.

The imaging system 100 may be provided, for example, in a vehicle movingin a predetermined place. The imaging system 100 may also behand-carried and portable. In one more preferred embodiment, the imagingsystem 100, particularly the lens 101 and the image sensor 102, may befixed in a predetermined position. Since the background does not changelargely when the subject is periodically imaged, a target signalcomponent contained in a differential signal is easily extracted in aflow of identifying the light-emitting location, which will be describedlater. The signals may be transmitted between the image sensor 102 andthe image processor 103 and between the image processor 103 and thedisplay 104 not only by wire but also via wireless communicationequipment (not shown).

This embodiment assumes the imaging conditions, for example, wherehydrogen leaked from a high-pressure hydrogen pipe of an outsidehydrogen station burns. The flame of the burning hydrogen emitsultraviolet light, which has, however, a lower intensity than backgroundvisible light. In addition, human eyes are not sensitive to ultravioletlight. It is thus usually difficult to detect the emission of the flameof the burning hydrogen with naked eyes.

To address the problem, this embodiment employs the image sensor 102with the predetermined sensitivities to both the ultraviolet light andvisible light. The image sensor 102 calculates the difference betweenthe image signals at different times to obtain an image signalassociated with the ultraviolet light.

FIG. 2 is a flowchart for identifying the location emitting ultravioletlight according to this embodiment.

At time t=t1, the subject S is imaged using the imaging system 100, inwhich the image sensor 102 outputs an image signal A1 to the imageprocessor 103 (step S1). In this embodiment, the subject S is ahigh-pressure hydrogen pipe or an assembly of such pipes laid in ahydrogen station.

At time t=t2 (t1≠t2), the subject S is imaged using the imaging system100, in which the image sensor 102 outputs an image signal A2 to theimage processor 103 (step S2). The image processor 103 generates adifferential signal A3 between the image signal A1 and the image signalA2 (step S3).

Next, it is determined whether or not the differential signal A3 isgreater than or equal to a predetermined value (step S4). If thedifferential signal A3 is smaller than the predetermined value, theprocess returns to step S1 to continue imaging the subject S. On theother hand, if the differential signal A3 is greater than or equal tothe predetermined value, the image processor 103 amplifies thedifferential signal A3 to obtain an amplified signal A4 (step S5). Ifthe differential signal A3 is greater than or equal to the predeterminedvalue, it can be said that there was a change in the intensity of thelight received at the image sensor 102 between the times t1 and t2. Forexample, assume that there is no hydrogen leakage from the hydrogen pipeat time t=t1 and hydrogen leaks out of the high-pressure hydrogen pipeand burns at time t=t2. The information on the hydrogen leakage is thenobtainable from the differential signal A3. In this case, thedifferential signal A3 and the amplified signal A4 are determined tocorrespond to the image signal mainly associated with the ultravioletlight emitted from the burning hydrogen.

Next, the image processor 103 adds the amplified signal A4 to the imagesignal A1 to obtain a composite image signal CI (step S6). Thedifferential signal A3 is assumed to be much smaller than the imagesignals A1 and A2. The differential signal A3 is thus amplified toobtain the amplified signal A4 to balance, for example, the luminancebetween the amplified signal A4 and the image signal A1 or A2 associatedwith the background image. The composite image signal CI is input to thedisplay 104, and the location emitting the ultraviolet light, that is,the location from which the hydrogen gas leaks, is identified in thehigh-pressure hydrogen pipe based on the displayed image (step S7).

According to this embodiment, the image sensor 102 is sensitive toultraviolet light and visible light. The image sensor 102 thus imagesthe location emitting the ultraviolet light and the backgroundsimultaneously to generate and take in image signals. The calculation ofthe difference between the image signals acquired at different timesallows for acquisition of the image signal associated only with theultraviolet light. Accordingly, the location emitting the ultravioletlight, that is, the location from which the hydrogen gas leaks in thisembodiment, can be identified. In addition, the composite image of theimage signal associated with the ultraviolet light and the image signalassociated with the background, that is, the image signal associatedwith the visible light is generated and analyzed. This facilitates theidentification of the location, from which the hydrogen gas leaks, inthe high-pressure hydrogen pipe.

As the lens 101, a lens correcting chromatic aberration for bothultraviolet and visible light is special and expensive. On the otherhand, a lens correcting no chromatic aberration is general and lessexpensive, and thus reduces the overall costs of the system.

In one preferred embodiment, the image signal A1 may be acquired at atime t1 when the pipe is surely normal. That is, the light coming fromthe subject S at the time t1 is mainly visible light, and the imagesignal A1 is an image signal mainly associated with the visible light.In this case, if the difference between the image signal An acquired atthe time to (n is an integer greater than or equal to 2) and the imagesignal A1 is equal to or smaller than a predetermined value, it isdetermined that the image signal An is also an image signal mainlyassociated with the visible light. Therefore, any of the image signalsA1 and An may be used to generate the composite image CI in step S6.

The intensity of the light from the background is different, forexample, between day and night, among seasons, or depending on weatheror other conditions. A plurality of images may thus be taken and thedata of the images may be stored with respect to these variations, whenthe pipe or pipe assembly is surely normal. The difference between animage signal and the image signals stored in advance may be obtained todetermine whether or not the value of the differential signal A3 isgreater than or equal to the predetermined value, depending on thecurrent season or the other conditions in daily monitoring.

In step S4, the imaging region may be divided into a plurality ofsubregions to determine whether or not the value of the differentialsignal A3 per unit area in each subregion is greater than or equal to apredetermined value. This allows for more reliable determination on thepresence or absence of the location emitting the ultraviolet light.

With a sufficiently high intensity, the differential signal A3 is notnecessarily amplified. In this case, the step S5 may be skipped so thatthe composite image signal CI may be generated from the image signal A1or A2 and the differential signal A3 in the step S6. Between the stepsS4 and S5, the differential signal A3 or the imaging region may bedivided into a plurality of subregions to determine whether or not themaximum value of the differential signal A3 per unit area in eachsubregion is greater than or equal to a predetermined value. If themaximum value is greater than or equal to the predetermined value, thestep S4 may be skipped.

In view of reducing the influence of the visible light such as externallight, the times t1 and t2 may be almost the same.

Second Embodiment

FIG. 3 is a functional block diagram of an imaging system according tothis embodiment. Different from the configuration in the firstembodiment, the configuration in this embodiment includes a focuscontroller 305, which controls the position of a lens (condenser) 301 toadjust the focal length. The lens 301 is a lens correcting no chromaticaberration. In this embodiment, a subject S is also a high-pressurehydrogen pipe or an assembly of such pipes laid in a hydrogen station.

The focus controller 305 is used to control the focal point of the lens301 in accordance with the wavelength of incident light. This allows formore accurate detection of the location emitting ultraviolet light.

FIG. 4 is a flowchart for identifying the location emitting ultravioletlight according to this embodiment.

First, the focal length of the lens 301 is adjusted using the focuscontroller 305 in accordance with ultraviolet light (step S11). StepsS12 to S16 are the same as the steps S1 to S5 in the flowchart in FIG.2. Detailed description thereof will thus be omitted. The image signalsB1 and B2, differential signal B3, and amplified signal B4 shown in FIG.4 correspond to the image signals A1 and A2, differential signal A3, andamplified signal A4 shown in FIG. 2, respectively. If the differentialsignal B3 is greater than or equal to a predetermined value, it can besaid that there was a change in the intensity of light received at theimage sensor 302 between the times t1 and t2. For example, assume thatthere is no hydrogen leakage from the hydrogen pipe at time t=t1 andhydrogen leaks out of the high-pressure hydrogen pipe at time t=t2. Theinformation on the hydrogen leakage is then obtainable from thedifferential signal B3. In this case, the differential signal B3 and theamplified signal B4 correspond to the image signal associated with theultraviolet light emitted from the burning hydrogen.

Next, the focal length of the lens 301 is adjusted using with the focuscontroller 305 in accordance with the visible light (step S17). At timet=t3, the subject S is imaged using the imaging system 300, in which theimage sensor 302 outputs an image signal B5 to the image processor 303(step S18).

Next, the image processor 303 adds the amplified signal B4 to the imagesignal B5 to obtain a composite image signal CI (step S19). Thedifferential signal B3 is assumed to be much smaller than the signal B5.The differential signal B3 is thus amplified to obtain the amplifiedsignal B4 to balance the luminance between the amplified signal B4 andthe image signal B5 associated with the background image. The compositeimage signal CI is input to the display 304, and the location emittingultraviolet light, that is, the position from which the hydrogen gasleaks, is identified in the high-pressure hydrogen pipe based on thedisplayed image (step S20).

According to this embodiment, the imaging is performed after adjustingthe focal length of the lens 301 in accordance with the ultravioletlight. This provides clear images associated with the ultraviolet lightat times t1 and t2. The imaging is also performed after adjusting thefocal length of the lens 301 in accordance with the visible light. Thisprovides a clear image associated with the visible light at time t3.Accordingly, in the composite image of these images, the imageassociated with the ultraviolet light is more clearly distinguishablefrom the image associated with the visible light. This leads to reliableidentification of the location emitting the ultraviolet light.

This will be described in more detail.

The differential signal B3 contains a signal component S1 associatedwith the visible light and a signal component S2 associated with theultraviolet light. The signal/noise (S/N) ratio is represented by thefollowing expression (1).

$\begin{matrix}\frac{S\; 2}{\sqrt{{S\; 1} + {S\; 2}}} & \left\lbrack {{Expression}\mspace{14mu}(1)} \right\rbrack\end{matrix}$

Here, the noise component as the denominator contains shot noise of thesignal component S1, which corresponds to the square root of S1, andshot noise of the signal component S2, which corresponds to the squareroot of S2. If the signal component S1 is much greater than the signalcomponent S2, the S/N ratio of the differential signal B3 decreases, asis apparent from Expression 1.

On the other hand, according to this embodiment, the image signal isacquired after adjusting the focal length of the lens 301 in accordancewith the ultraviolet light. The visible light component is thus takenout of the subject S into the light receiving surface (not shown) of theimage sensor 302, while not being focused. That is, the intensity of thevisible light incident on each pixel of the image sensor 302 decreases.As viewed in each pixel unit, the signal component S1 contained in thedifferential signal B3 can decrease to improve the S/N ratio of thedifferential signal B3. This increases the intensity of the signalassociated with the ultraviolet light in the composite image signal CI,and thus allows for reliable identification of the location emitting theultraviolet light.

While the focal point of the lens 301 is adjusted using the focuscontroller 305 in the second embodiment, the adjustment may be performedmanually. The amount of focus adjustment of the lens 301 with respect tothe ultraviolet light may be estimated in advance based on thewavelength of ultraviolet light and the design parameters of the lens301. This amount may be used when driving the focus controller 305.

In this embodiment, the image focused on the ultraviolet light isacquired at time t1, and then the image focused on the visible light isacquired at time t3. The image focused on the visible light may beacquired before the image focused on the ultraviolet light. That is, thetime t3 may be prior to the time t1.

An example has been described where the imaging systems according toEmbodiments 1 and 2 are applied to monitoring of hydrogen gas leakage orresultant fire in a hydrogen station. The imaging system according tothe present invention is also applicable for other purposes. Forexample, the imaging system is also applicable to discharge detection ina power plant.

The imaging system according to the present invention detects weakultraviolet light even in a bright place such as outdoors, and is thusapplicable to monitoring of a malfunction in a facility.

DESCRIPTION OF REFERENCE CHARACTERS

-   -   100, 300 Imaging System    -   101, 301 Lens (Condenser)    -   102, 302 Image Sensor    -   103, 303 Image Processor    -   104, 304 Display    -   305 Focus Controller    -   A1, A2, B1, B2, B5 Image Signal Acquired at Predetermined Time    -   A3, B3 Differential Signal Associated with Ultraviolet Light    -   A4, B4 Amplified Signal Associated with Ultraviolet Light    -   CI Composite Image Signal (First or Second Composite Image        Signal)    -   S Subject

What is claimed is:
 1. An imaging system comprising: an image sensorsensitive to ultraviolet light and visible light; a lens configured tofocus light from a subject onto the image sensor; and an image processorconfigured to process image signals output from the image sensor,wherein the image processor acquires a differential signal between animage signal output from the image sensor at a first time underillumination containing the visible light and an image signal outputfrom the image sensor at a second time under illumination containing thevisible light, determines that the differential signal contains a signalassociated with the emission of ultraviolet light if the differentialsignal is greater than or equal to a predetermined value, and generatesa first image signal based on the differential signal.
 2. The imagingsystem of claim 1, wherein the image processor further generates a firstcomposite image signal of the image signal output from the image sensorat the first time and the first image signal or an amplified first imagesignal.
 3. The imaging system of claim 1, further comprising: a focuscontroller configured to control a position of the lens to adjust afocal length of the lens with respect to light with differentwavelengths, wherein the focus controller controls the position of thelens to match the focal length of the lens to the ultraviolet light atthe first and second times, and controls the position of the lens tomatch the focal length of the lens to the visible light at a third timeunder illumination containing the visible light, and the image processorfurther generates a second composite image signal of an image signaloutput from the image sensor at the third time and the first imagesignal or an amplified first image signal.
 4. The imaging system ofclaim 2, further comprising: a display configured to display an image ofthe subject based on the first composite image signal.
 5. The imagingsystem of claim 3, further comprising: a display configured to displayan image of the subject based on the second composite image signal. 6.The imaging system of claim 1, wherein the ultraviolet light has awavelength within a range from 200 nm (inclusive) to 400 nm (exclusive),while the visible light has a wavelength within a range from 400 nm to700 nm.
 7. The imaging system of claim 2, wherein the ultraviolet lighthas a wavelength within a range from 200 nm (inclusive) to 400 nm(exclusive), while the visible light has a wavelength within a rangefrom 400 nm to 700 nm.
 8. The imaging system of claim 3, wherein theultraviolet light has a wavelength within a range from 200 nm(inclusive) to 400 nm (exclusive), while the visible light has awavelength within a range from 400 nm to 700 nm.
 9. The imaging systemof claim 4, wherein the ultraviolet light has a wavelength within arange from 200 nm (inclusive) to 400 nm (exclusive), while the visiblelight has a wavelength within a range from 400 nm to 700 nm.
 10. Theimaging system of claim 5, wherein the ultraviolet light has awavelength within a range from 200 nm (inclusive) to 400 nm (exclusive),while the visible light has a wavelength within a range from 400 nm to700 nm.
 11. A method of identifying a location emitting ultravioletlight using an imaging system including at least: an image sensorsensitive to ultraviolet light and visible light; a lens configured tofocus light from a subject onto the image sensor; and an image processorconfigured to process image signals output from the image sensor, themethod comprising: imaging a subject to acquire ones of the imagesignals at a first time under illumination containing the visible lightand a second time under illumination of the visible light using theimage sensor; obtaining a differential signal between the ones of theimage signals acquired at the first and second times using the imageprocessor; determining whether or not the differential signal is greaterthan or equal to a predetermined value using the image processor;determining that the differential signal contains a signal associatedwith the ultraviolet light if the differential signal is greater than orequal to a predetermined value; generating a first image signal based onthe differential signal containing the signal associated with theemission of ultraviolet light using the image processor; and identifyingthe location associated with the emission of the ultraviolet light inthe subject using the image processor, based on the first image signal.12. The method of claim 11, wherein the location emitting theultraviolet light is identified in the subject using the imageprocessor, based on a first composite image signal of the first imagesignal or an amplified first image signal and the one of the imagesignals acquired at the first time.
 13. The method of claim 11, furthercomprising: adjusting a position of the lens to match a focal length ofthe lens to the ultraviolet light at the first and second times;adjusting the position of the lens to match the focal length of the lensto the visible light at a third time under illumination containing thevisible light; and imaging the subject to acquire an image signal at thethird time using the image sensor, wherein the location of the emissionof the ultraviolet light is identified in the subject using the imageprocessor, based on a second composite image signal of the first imagesignal or an amplified first image signal and the image signal acquiredat the third time.
 14. The method claim 11, wherein the ultravioletlight has a wavelength within a range from 200 nm (inclusive) to 400 nm(exclusive), while the visible light has a wavelength within a rangefrom 400 nm to 700 nm.
 15. The method claim 12, wherein the ultravioletlight has a wavelength within a range from 200 nm (inclusive) to 400 nm(exclusive), while the visible light has a wavelength within a rangefrom 400 nm to 700 nm.
 16. The method claim 13, wherein the ultravioletlight has a wavelength within a range from 200 nm (inclusive) to 400 nm(exclusive), while the visible light has a wavelength within a rangefrom 400 nm to 700 nm.
 17. The imaging system of claim 1, wherein theimaging of the subject using the image sensor is continued if thedifferential signal is smaller than a predetermined value.
 18. Themethod of claim 11, further comprising: determining, by the imageprocessor, whether or not the differential signal is equal to or greaterthan the predetermined value, and continuing with the imaging of thesubject using the image sensor if the differential signal is smallerthan the predetermined value.